In the field of liquid crystal display devices (LCD), organic EL display devices (OLED), particularly portable display devices such as mobile or portable phones, weight saving and reduction in thickness of display devices have been important objects.
To accomplish the above objects, it has been studied to make a glass substrate to be used for display devices thinner, but if the glass substrate is made thin, a decrease in strength becomes problematic, and the amount of deformation tends to be significant, and accordingly such is not applicable to the existing production line.
Accordingly, a process has been proposed to produce a display device in such a manner that in order to reinforce strength of a glass substrate having a small plate thickness (hereinafter referred to as a “thin plate glass substrate”) and to secure a plate thickness applicable to the existing production line, predetermined treatment is applied to produce a display device in a state of a laminate (thin plate glass laminate) obtained by bonding the thin plate glass substrate to another support glass substrate, and after completion of the treatment, the thin plate glass substrate and the support glass substrate are separated (Patent Documents 1 to 6).
In such a process for producing a display device, as a method of laminating and fixing the thin plate glass substrate and the support glass substrate, a method of fixing them by means of electrostatic adsorptivity or vacuum adsorptivity caused between the glass substrates (e.g. Patent Document 1), a method of fixing both ends of the glass substrates by means of glass frit (e.g. Patent Document 2), a method of fusing the two glass substrates by irradiating a portion in the vicinity of the edge surfaces of peripheral portions with laser beams (e.g. Patent Document 3), and a method of disposing a removable adhesive or adhesive sheet over the entire surfaces of the glass substrates, and fixing them by the adhesive force (e.g. Patent Documents 4 to 6) have been proposed.
These methods have latent problems which may adversely affect a display device to be produced.
That is, by the method of fixing the glass substrates by means of electrostatic adsorptivity or vacuum adsorptivity, the method of fixing both ends of the glass substrates by means of glass frit, or the method of fusing the two glass substrates by irradiating a portion in the vicinity of edge surfaces of peripheral portions with laser beams, it is difficult to avoid inclusion of bubbles or convex defects due to foreign matters such as dust in the process of laminating and closely bonding the glass substrates without any interlayer, and accordingly it is difficult to obtain a glass substrate laminate having smooth surface.
In the case of the method of disposing a removable adhesive or adhesive sheet over the entire surface of the glass substrates, it is easy to avoid inclusion of bubbles as compared with the case of directly laminating the glass substrates, and it is considered that convex defects due to foreign matters are less likely to occur. However, it is difficult to separate the thin plate glass substrate and the support glass substrate, and the thin plate glass substrate may be broken at the time of separation. For example, by inserting a razor blade between the thin plate glass substrate and the support glass substrate, it is possible to initiate separation of these substrates, but the glass substrates may be damaged by the razor blade. Further, the thin glass substrate having small thickness may be broken in the step of separating these substrates. Further, remaining of the adhesive on the thin plate glass substrate after the separation is also problematic. Further, the display device production process comprises a step which requires treatment at high temperature, such as a step of firing an insulating film or an alignment film in a process for producing a liquid crystal display device. Therefore, heat resistance for a display device is required for the adhesive or adhesive sheet, but a method which satisfies both heat resistance and removability has not been proposed yet.
Patent Document 1: JP-A-2000-241804
Patent Document 2: JP-A-58-54316
Patent Document 3: JP-A-2003-216068
Patent Document 4: JP-A-8-86993
Patent Document 5: JP-A-9-105896
Patent Document 6: JP-A-2000-252342